Ординатура / Офтальмология / Английские материалы / Glaucoma - Basic and Clinical Concepts_Rumelt _2011

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11

Corneal Viscoelastical

Properties Related to Glaucoma

Horea Demea1, Sorina Demea1 and Rodica Holonec2

1Ophthalmological Center - Review, Cluj-Napoca

2Technical University of Cluj-Napoca

Romania

1. Introduction

Although elevated IOP is clearly the most frequent causative risk factor for glaucomatous optic nerve atrophy, it is not the only factor, and attempts to define glaucoma on the basis of ocular tension are no longer recommended. Considering IOP as a risk factor in the appearance of the glaucomatous lesions at the level of the optic nerve, it’s important to establish the mechanisms which may enhance or reduce this risk. These complex processes refer to the way in which the IOP is transmitted from the ocular structures to the optic nerve head. (Downs et al., 2005)

For a better understanding of our work, some physical parameters must be defined and known. We found them used in materials physics: stress, stretch, strain, deformability, elastic and viscous materials, etc. A short special subchapter is dedicated to these mechanical characteristics.

The Reichert device named Ocular Response Analyzer (O.R.A.) is a special device used to measure intra ocular pressure correlated with the viscoelastic properties of the cornea – named IOPcc. In order to monitor the influence of IOPcc on the optic nerve we also made measurements on the Retinal Nerve Fiber Layers (RNFL). For that purpose, we used a Zeiss device named Stratus OCT on the Optical Coherence Tomography.

Using clinically measured values with Ocular Response Analyzer for IOPcc and the computed value which describes the performance, the efficiency (%) of eliminating the overpressure /for cycle of loading – named by us the specific damping capacity (φ) - we made a graphical representation of these two parameters, to grade the particular effort of the ocular structure and implicitly of the retinal nervous fibers layer of the studied patient. We named this graphic the Effort Staging System (ESS) (Demea et al., 2008) and we use it to classify glaucoma risk for our patients. In order to make our work easier we developed a computerized application which practically takes the data measured by us and calculates the specific damping capacity, so the patient is automatically introduced in the ESS scale. Consequently we obtain the class of the glaucoma risk, damage effort of RNFL, where that patient is situated.

The application has a user-friendly interface and helps the ophthalmologists in the clinical diagnosis.

2. Physical definitions – The dynamic of the expansion effort of the ocular structures

For a better understanding of the viscoelastic properties of the cornea we considered important to mention some elements from the material physics that help to characterize the properties of the ocular structures from the mechanical point of view.

2.1 Materials resistance to deformation

The Materials resistance to deformation is studied in accordance with two parameters: stress (σ) and strain (ξ).

The stress (σ) is defined as a measure of internal forces that arise in a body being deformed as a result of external forces and the stress intensity is given by the force which operates on the surface unity dF/dS =dP.

The strain can be characterized by more parameters. Thus, the stretch (dS) represents the expansion difference the material can reach, after the stress was applied. If the stretch ratio dS/S increases, the surface becomes elastic and can be deformed. If dS/S decreases, the surface becomes rigid and its deformability decreases as well.

At the same time, we can talk about the deformation effort dP/dS which can increase either due to the stress growth (dP), or due to the surface rigidity (dS decreases).

2.2 The viscoelasticity of the soft tissues

From the perspective of the stress factor, if we take into account the time factor (t) and its application frequency (f), the materials have an elastic or viscose behavior.

In the case of an elastic behavior the deformation is independent of the stress factors “t” and “f”. For example, an elastic arch submitted to the compression is deformed in a linear way, according to the applied force size, and this deformation doesn’t depend on the application time or on the behavior. The releasing of the tension takes place linearly as well, while the stress intensity decreases. Elastic materials strain instantaneously when stretched, and just as quickly return to their original state once the stress is removed.

In the case of a viscose behavior, the strain depends on the application time and/or on the application frequency. In this way, the applied stress resistance is mainly dynamically dependent on the force application speed, high speed means great resistance. Viscous materials resist shear flow and strain linearly with time when a stress is applied.

Viscoelasticity is the materials’ property to exhibit both viscous and elastic characteristics when undergoing deformation. Cornea and all the soft tissues are viscoelastic and their response to the stress is a combination between an instantaneous response (elastic) and a response dependent on time (viscous – reaction time latency). Viscous-elastic materials have elements of both of these properties and exhibit time dependent strain (Jonas & Budde, 2000).

3. Ocular Response Analyzer (ORA) and the calculation of specific damping capacity

From the actual instruments for intraocular pressure measurement we chose the non-contact tonometer called Ocular Response Analyzer (Figure 1). This device determines 2 types of parameters:

•the ones related to the “stress” generated by the increase of the intraocular pressure named: IOP cc – intraocular pressure corrected and IOPg – intraocular pressure according to Goldman norms.

•the ones related to the “material behavior ”, such as cornea, submitted to this stress, named: CH – Corneal Hysteresis and CRF - Corneal Resistance Factor

Fig. 1. Ocular Response Analyzer

The producer of ORA specified that:

1.IOPgis the real IOP measured by the instrument, dependent on the cornea biomechanics.

2.IOPcc - this is the intraocular pressure measurement that is less affected by corneal properties than other methods of tonometry, such as Goldman. The instrument automatic adjusts the IOPg to IOPcc. The recommendation of Reichert Company is for use in clinical evaluation of intraocular pressure the value of IOPcc instead of IOPg.

3.CH – is the pressure lost during an upload cycle. This parameter is a measure of corneal tissue properties, a result of viscous damping in the corneal tissue.

The normal values for IOPcc and CH are shown in Table 1, as in (Allingham & al., 2005; Kevin & al., 2004; Luce &Taylor, 2006).

Table 1. Normal values for IOPcc and CH

For example, the situation in which IOPcc < 20 mmHg and CH > 10 mmHg is a normal one, where cornea and the other ocular structures have functional reserves, which are able to absorb the loaded IOP and to reduce the tensional stress impact on the posterior pole (optic nerve). The situations where IOPcc reaches at values > 20 mmHg and CH ≤ 10 mmHg represent the “balance break point”, that is, the moment where cornea loses the compensation capacity of the tensions and the “loaded stress” is transmitted to the ocular posterior pole.

To detect this situation of balance break point, we calculate the efficiency of eliminating the overpressure / cycle of loading and we named this: Damping capacity (φ). The damping capacity can specifically be calculated as the ratio between the lost energy and the stocked energy during a loading cycle.

Normal values for specific damping capacity (φ) are over 30 %.

O.R.A. may offer deductive, indirect data on the ocular structures strain (σ) characteristics. Thus, for instance, a low specific damping capacity (φ) is characteristic for the “rigid” viscoelastic systems, and this situation frequently occurs in glaucoma because of the nonenzymatic glycation of the glucose with the collagen fibers, which leads to sclera stiffness.

4. Cornea – The specific damping capacity in glaucoma etiopathogeny

It has been long suspected that corneal biomechanical properties influence the results and outcomes of various ocular measurements and procedures, and may hold clues to diagnosing and managing ocular diseases. Human corneal tissue is a complex viscouselastic structure (Ethier et al., 2004). Almost all known glaucoma evaluating systems consider an elastic ocular model, where IOP measured at the anterior eye segment is totally transmitted to the posterior eye pole. We consider the viscous-elastic ocular model (Sigal et al., 2004), where only a part of this IOP is considered to be transmitted toward posterior pole, because of the specific damping capacity or partial absorption of the pressure in the ocular walls and other ocular structures. A high damping capacity (more vascoelastic ocular structures) reduces the risk of the glaucoma, by decreasing the transmitted pressure toward optic nerve in the posterior pole. In reverse order, a reduced damping capacity (a more rigid eye, for example - an aged person) increases the risk of the optic nerve damage even at the medium IOP.

We studied the correlation between the most important factors in diagnosis and evaluating a glaucoma suspect patient: IOPcc – as a loading factor (stress) and CH – as unloading factor (protective). We noticed that it is most important to appreciate the efficiency of pressure elimination depending on each cycle of charging, reason for which we calculated the specific damping capacity (φ).

5. Evaluation method of the pressure risk in glaucoma

In order to quantify the mechanical risk of lesion in glaucoma we emphasized (Demea et al., 2008) the need to determine two important parameters from a pathophysiological point of view: the IOPcc - Compensated Intraocular Pressure and the CH - Corneal Hysteresis. We used O.R.A to measure these values, the most important factors in diagnosing and